According to the USB standard, the address field for addressing a USB port is limited to seven bits. This limits a USB host to connecting to no more than 127 devices (i.e., 27=128 minus one allocated for enumeration). As computer systems become more complex, this limitation can cause problems with a computer that needs to connect to greater than 127 USB devices.
Once solution would be to change the standard to increase the number of address bits. However, older devices have been built to the seven bit address standard and would be unable to use devices that were designed to a new standard.